Picture tubes for three-colour television systems comprising deflection coils



July 12, 1960 Q J, HAANTJES ETAL 2,945,157

PICTURE TUBES FOR THREE-COLOUR TELEVISION SYSTEMS COMPRISING DEFLECTION COILS Filed Oct. 16, 1958 4 Sheets-$119M. 1

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\ INVENTOR JOHAN HAANTJES GERRIT JAN LUBBEN AGEN July 12, 1960 J. HAANTJES EI'AL I 2,945,157

PICTURE TUBES FOR THREE-COLOUR TELBVISIO v SYSTEMS COMPRISING DEFLECTION cons Filed Oct. 16, 1958 4 Sheets-Sheet 2 Y i 1 v i I f "'T 7A {-2.

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INVENTOR JOHAN HAANTJES GERRIT JAN LUBBEN FIGS if K W AGENT July 12, 1960 J. HAANTJES ET 2,945,157

- PICTURE TUBES FOR THREE-COLOUR TELEVISION SYSTEMS COMPRISING DEFLECTION cons Filed Oct. 1a, 1958 4 Sheets- Sheet s FIG 8b INVENTOR JOHAN HAAN TJES GERRI 1 JAN LU BBEN BY wi- AGENT July 12, 1960 J. HAANTJES ETAL ,9

PICTURE TUBES FOR THREE-COLOUR TELEVISION v SYSTEMS COMPRISING DEFLECTION COILS Filed Oct. 16, 1958 45heets-Sheet 4 INVENTOR JOHAN 'HAANTJES GERRIT JAN LUBBEN a I /Q.

AGENT United States Patent PICTURE TUBES FOR TrmaE-coLoU-n TELE- 'VISION SYSTEMS COMPRISING DEFLEC- TION cons Iohan Haantjes and Gerrit Jan Lubben, Elndhoven, Netherlands, assignors to North American Philips Company Inc., New York, N.Y., a corporation of Dela- This invention relates to picture tubes for threecolour television systems comprising deflection coils and a substantially flat screen which is substantially at right angles to the axis of the tube, and in which three electron beams, emanatingfrom three electron guns arranged at the corner points of a triangle located in a plane parallel to and opposite the said screen, are deflected in two directions substantiallyat right angles to each other.

As is well-known, for colour television purpos s, the three beams are controlled so that each beam strikes a determined phosphor element provided on or behind the screen, whereby each element fluorcsces in another colour.

In the so-called mask tube, said control is effected in a manner such that the three beams each pass through an aperture of the mask at relatively different angles of incidence and then strike the blue, red and green phosphor elements provided in punctiform on the plate located behind the mask. 1 The term screen is to be understood in this case to mean the above-mentioned mask which must be struck by the three beams in each case at substantially the same point. 'It is known that, even though it is ensured by means of static convergency that the three beams strike the mask at one point if no deflection currents flow through the deflection coils, the beams upon being deflected by no means overlap each other on the mask as a result of the aberration faults then occurring.

In known systems, this disadvantage is obviated by the use of dynamic convergence, whereby by means of additional deflection coils and additional convergence currents the beams are influenced individually and caused to overlap on the mask, independent of the extent of the deflection. Consequently, in this case the use of three separate convergence circuits and three additional sets of deflection coils is required, which is expensive and causes difliculty in adjustment.

It has therefore previously been suggested to manufacture picture tubes in which the three electron guns are located in a plane containing the axis of the tube and in which the deflection of the three electron beams in one of the two directions is parallel to this plane. With coils of proper structure, it is thus possible to cause the three electron beams substantially to overlap throughout the mask of the picture tube, but such positioning of the electron guns is attended with the configuration of the phosphor elements provided on the plate behind the mask being such that the picture to be reproduced acquires a linear structure, while also the space in the neckof the picture tube must be larger than if, as is usually the case, the three guns are positioned, at the corner points of an equilateral triangle.

The picture device according to the invention mitigates this disadvantage and is characterized in that for the direction of deflection which is parallel t'o' a plane containing two of the three guns, the meridionalimage plane substantially coincides with the screen of the picture tube and for the other direction of deflection the 2,945,157 Patented July 12, 1960 'ice sagittal image plane substantially coincides with said screen.

A further embodiment of the said picture device is characterized in that, by means of dynamic convergency, exerted only upon the electron beam produced by the gun located outside the plane containing the said two other guns, it is ensured that this electron beam, during deflection throughout the screen, on the screen overlaps the two other electron beams which, due to the coinciding of said image planes with the screen of the picture tube, are caused to overlap each other on the screen, independent of the extent of the deflection.

In order that the invention may be readily carried into eifect, one embodiment will now be described in detail, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows diagrammatically a deflection system in which the points of incidence of the three electron beams and the resultant pictures are shown;

Fig. 2 shows a possible embodiment of the deflection coils for deflection in the vertical direction;

Fig. 3 shows a portion of the field strength to be described hereinafter, which is produced by means of the coils of Fig. 2;

Fig. 4 shows a possible embodiment of the deflection coils for deflection in the horizontal direction;

Fig. 5 shows a portion of the field strength to be described hereinafter, which is produced by means of the coils of Fig. 4;

Figs. 6, 7 and 8 show other embodiments of the coils in horizontal and vertical directions, and

Figs. 9 and 10 show possible circuit diagrams for connecting the coils of Figs. 7 and 8.

Referring now to Fig. 1, this figure shows a rectangular system of co-ordinates, wherein the z-axis represents the axis of the picture tube and the positive z-direction is the direction in which the electrons are travelling, whereas x and y are the directions in which the electrons are deflected. In this figure, it is assumed that the electron beams B, R and G produced by three electron guns, which must strike the blue, red and green phosphor elements respectively, are not yet deflected at z=Z It is also assumed that, by means of focussing and static convergence, the three beams are adjusted so as to strike the mask of the picture tube at the same area, that is to say at the area x=0, y=0, z=Z if no current flows through the deflection coils.

v The points of incidence of the beams at the area z=Z are also marked with B, R and G in Fig. 1 and from this figure it appears that, when reckoning with polar co-ordinates, it is possible to indicate the co-ordinates of incidence in the plane z=Z by:

The mask is represented diagrammatically by lines 1, 2, 7 and '10 and is located in the plane 2:2 at right angles to the axis of the tube. When the three beams are If there is deflection only in the x-direction and if the error in this direction has been reduced to zero (that is to say Ax =Ax =Ax then there is developed a line instead of a point which line by definition is called a meridional image or focal line and the plane containing this meridional focal line is called the meridional image plane.

If, however, upon deflection in the x-direction, the error in the y-direction has been reduced to zero (that is to say Ay =Ay =Ay =O) then there is developed a line instead of a point which line by definition is called a sagittal image or focal line and the plane containing this sagittal focal line is called the sagittal image plane.

Similarly, when upon deflection in the y-direction the error in the x-direction has been reduced to zero, the term sagittal focal line and, if the error is zero in the ydirection, the term meridional focal line will be used.

Thus, in Fig. 1, in which also a fourth beam emanating point P is shown for the sake of clarity, upon deflection in the positive x-direction, line 5 is the meridional focal line (that is to say 5 is the position of the beams B, R, G and P located in the meridional image plane) and line 6 is the sagittal focal line (that is to say 6 is the position of the beams B, R, G and P located in the sagittal image plane) whereas upon deflection in the negative y-direction, line 8 is the sagittal focal line and 9 is the meridional focal line.

Thus one can say:

(1) In the sagittal image plane the focal line is located in the direction of deflection,

(2) In the meridional image plane the focal line is located at right angles to the direction of deflection.

As a rule, the sagittal and meridional image planes are curved, non-coincident surfaces. Hereinafter it will be shown that if, according to the invention, for deflection in the x-direction which is parallel to the plane containing the red and green guns:

(1) The curvature of the meridional image plane is reduced to zero;

(2) This meridional image plane is caused to coincide with the mask;

(3) The so-called coma errors are substantially reduced; the red and green beams strike the mask at the same areas independent of the deflection in the x-direction, so that only the blue beam must be caused to overlap on the screen the red and green beams by means of dynamic convergence.

Similarly, it can be shown that if, according to the invention, for deflection in the y-direction:

(1) The curvature of the sagittal image plane is reduced to zero;

(2) This sagittal image plane is caused to coincide with the mask;

(3) The so-called coma errors are substantially reduced; the red and green beams strike the mask at the same areas independent of the deflection in the y-direction, so that only the blue beam must be caused to overlap on the screen the red and green beams by means of dynamic convergence.

Upon deflection in a magnetic field, so-called astigmatic errors occur, which may be determined with the aid of the formulae:

(See for the derivation of said formulae 1. Haantjes, J. and G. J. Lubben, Philips Research Report 12, pages 46- 48, February 1957) wherein A and 8,, (n=4, 5, 6) are integral functions of the field strength and of the deflection, X is the extent of deflection in the x-direction at the area z=Z and Y is the extent of deflection in the y-direction at the area 2 2,.

2,, Z,, r and are the co-ordinates shown in Fig. 1.

When the co-ordinates of incidence for the blue beam are substituted in the Formulae 3 and 4, the aberrations from the Formulae la and 2a are:

Similarly, by substituting the co-ordinates of incidence of the red and green beams, the other aberrations are found, viz.:

Considering only the deflection in the y-direction, then Y 7 0 and X,=0, so that the Formulae 5 and 6 change to:

From the Formulae 5' and 6 it may be seen that, if B =0, the aberration in the x-direction becomes zero and only an aberration in the y-direction subsists. According to the definition given hereinbefore, this is the sagittal focal line and since the radius of curvature of the it follows therefrom that, by causing B to approach zero, the sagittal image plane changes to a flat plane which coincides with the mask of the picture tube.

Now, an aberration in the y-direction remains, but this is the same for the red and green beams so that these beams strike the mask at the same points independent of the values of B and Y By influencing the blue beam in proportion to Y}, this beam may also be caused to strike the mask at the same points as the red and green beams. It thus suffices to pass a parabolic current of the raster frequency through one set of additional deflection coils (if Y, is the deflection in vertical direction) and the field of said coils must then act only upon the blue beam and is directed in parallel to the x-axis.

For the x-direction Y and X o, so that the aberrations are:

If A =0, the aberration in the x-direction is again reduced to zero and, despite the aberration in the y-direction, the red and green beams strike the mask at the same areas independent of the values of A and X The blue beam can again be caused to overlap the red and green beams by passing a parabolic current of line frequency (if X, is the deflection in a horizontal direction) through a second set of additional deflection coils, and said coils also must produce a field directed in parallel to the x-axis. Since in either case a field is to be produced directed in parallel to the x-axis, it suflices to use one set of additional coils which must be traversed by a current proportional to both X, and Y}.

Since, in the last-mentioned case, only an aberration in the y-direction remains, whereas there is deflection in the x-direction, a meridional focal line is nowconcerned. The radius of curvature of the meridional image plane, in which the said meridional focal line is situated, may be represented approximately by the formula so that this meridional image plane also changes to a flat plane, if A 0, which flat plane coincides with the mask of the picture tube.

If there is deflected in both the x-direction and the ydirection, all that has been described above remains true, but there is an additional aberration in both the xand ydirections as a result of most of the right-hand terms in the Formulae 3 and 4. This aberration may .always bereduced to zero by providing that the term A +B =0.

During deflection there also occur so-called coma-errors, which may be determined with the aid of the formulae:

H and H are the coeflicients of the power series,

H x= 1o+ I2y ro( %1 wherein H is the field strength in the x-direction in the plane x=0. H has the same sign (in this case assumed as positive) along the z-axis between the values 2:2 and z=Z whereas the sign of H varies between the same values as a function of z. In this case z=Z is the beginning and z=Z is the end of the deflection coil system, k is a proportionality constant and the terms Y and Y represent the extent of the deflection and the variation thereof in the y-direction as given by the relations Y=Y (z) and for which there applies: Y=Y'=0' if 2:2 The first two terms of (9) provide together a positive contribution, so that the third term must provide a negative contribution. For a positive H Y is negative with the direction of propagation of the electrons, chosen in this example, so that, since the contribution of the deflection is maximum at the end of the coils, the contribution of H must also be negative at the end of the coils.

For obviating the astigmatism in the x-direction, it is necessary that A =0 or:

wherein H is the field strength in the y-direction in the plane y=0. For H there applies, under similar conditions as for H that the sign along the z-axis remains the same (in this case also assumed as positive) and um der similar conditions as for H there applies that the sign of H varies as a function of 1, while X and X represent the extent of the deflection and its variation in the x-direction as given by: X ==X (z) and dX dz for which there applies: X'=X if z=Z The first term of (11) provides a positive contribution so that the second term must provide a negative contribution. For a positive H X is positive with the directions chosen in this example so that H must be positive at the end of the coils.

The conditions A +B =0 is satisfied if The first two terms provide a positive contribution so that the last two terms must provide a negative contribution.

Now, Y and Y are negative at the end of the coils whereas X, is positive. According to the foregoing, H must be positive at this area, so that the total contribution of the third term is positive. For the fourth term there applies X and X positive, Y negative, while H12, as explained above, is negative. The fourth term thus provides a negative contribution and if the sum of the third and fourth terms is to be negative, it is necessary that ml In order to obviate the coma errors, it is necessary and sufficient that the terms A =A =B =B =0. Now:

Consequently, the condition B 1=Bg=0 can never be fulfilled but a satisfactory result is obtained if If for H a function is chosen as shown in Fig. 3, the above-mentioned condition is satisfied, while H provides a negative contribution towards the end of the coils (z=Z Such a course of the field strength may be obtained with a set of coils as shown in Fig. 2. These coils serve to deflect in the y-direction and for deflection in the negative direction they must convey a current i, the direction of which is indicated by arrows. An H field is produced as given by the Formula and an H field as shown in Fig. 3, since there applies with great approximation:

wherein p; and ,0; are the magnitudes indicated in Fig. 2.

that 30.

In a similar manner it is found from ZIHII2(Z Z')3 A 2 k d2 (15) s== 1 A1 8 that the condition Aq=A8=0 cannot be satisfied, but that in this case also it sufiices if For this purpose, a function must be chosen for H as shown in Fig. 5. In this case the last-mentioned integral becomes zero and, furthermore, H provides a positive contribution towards the end of the coils (z=Z The course of the field strength shown in Fig. 5 may be obtained with a set of coils as shown in Fig. 4. For this set of coils the same is true as for the coils of Fig. 2. An H field is produced in accordance with Formula 12 and an H field as shown in Fig. 5, since there applies with great approximation:

wherein p and ,l/ are the magnitudes indicated in Fig. 4. The conditions are fulfilled if h 0 in the vicinity of z=Z h 0 in the vicinity of z=Z and again h =0 approximately midway between Z and Z For this there applies =30 at the centre, 30 for z- Z and b 30 for z in the vicinity of Z Furthermore, it is necessary to fulfill the condition which may be achieved if for approximately equal values for H and H the angles 1/; and #1 are chosen so that there always applies |h l [h For the sake of completeness, it is to be noted that the shape of the halves of the coils is dependent upon the radius p, upon the length of the coils Z Z and upon the distance to the mask Z -Z If t, is called 41;, at the area 2,, and \[1 at the area Z and if the angle p is called rp at the area Z and 30112 at the area Z then each half of the coil for the vertical deflection (y-direction) at a distance Z,-Z =44 cms. from the screen of the tube acquires the dimensions:

Z -Z =12.5 cms.; p;=3 cms.

For the horizontal (x-direction) this is:

Z Z =12.5 cms.; p =3 cms.

m= l II2= Another embodiment is shown in Figs. 6, 7 and 8.

In these figures, 11 and 12 indicate annular ferromagnetic cores, which may be arranged around the neck of a picture tube (40) and on which coils 13 to 20 are toroidally wound which serve for deflection in the vertical direction and coils 21 to 28 which serve for deflection in the horizontal direction. Ring 12 is closer to the screen than ring 11. In Fig. 6 the z-axis is also the axis of the picture tube, the coils 13, 14, 17 and 18 only being shown in full lines and the coils 21, 22, 25, and 26 in dashed lines. The three guns are shown at the left, with the blue gun 41 located on the Y axis, and the red gun 42 below it. The green gun 43 (see Fig. l) is in line with and behind the red gun 42 and will not be seen.

Figs. 7 and 8 show cross-sections of the cores 11 and 12 and in these cross-sections the orientations of the coils and the directions of the currents are shown. Thus, the (x) sign indicates that the current at this area flows in the direction of the positive z-axis, whereas a sign indicates that the current at this area flows in the direction of the negative z-axis.

For these coils also there apply the formulae:

wherein t is half the acute angle between two coils having the same direction of current.

and h in Fig. 7a, =2s and in Fig. 7b ,=43, whereas in Figs. 8a and 8b the different angles are given by:

For these values again the condition is fulfilled: |h |h and the functions H and H are obtained as shown in Figures 3 and 5. Since the cores 11 and 12 contain all the coils, for this system of deflection coils p =p =p, wherein p is the mean radius of the cores.

A possible method of circuiting the various coils for deflection in the vertical direction is shown in Figs. 9 and 10. In these latter figures, the coils are represented by inductor symbols and given the same reference numerals as in Figs. 6 to 8. Figs. 6 to 8 show the structural arrangement of the coils relative to the tube, whereas Figs. 9 and 10 show suitable circuit arrangements for energizing the coils.

In Fig. 9, the coils 13 to 16 arranged on the core 11 are connected in series, as are the coils 17 to 20 arranged on the core 12. The two series-branches are connected together at their lower ends to a terminal 32 and at their upper ends, via the variable inductance 30, to a terminal 29.

By shifting a core 31, the sawtooth current supplied to the terminals 29 and 32 may be distributed over the two branches in an arbitrary manner. It will be evident that the connection to the coils must be such that the direction of the current in each part has the proper polarity in the direction of the z-axis. The coil 30 is required to be adjusted so that it does not act upon the deflection of the beams.

Fig. 10 shows another method of connecting the same coils. In this case the two series-branches are connected in series, a tapping between the coils 16 and 17 being connected via a lead 36 to a tapping on a variable inductance 34. The sawtooth current is supplied to terminals 33 and 37 and, by shifting a core 35, the current may be distributed arbitrarily over the various branches so that the correct number of ampere turns for each branch may be obtained.

The method of connecting for deflection in the horizontal direction may be similar in either case.

It is to be noted that, in the example under consideration, the red and green beams always coincide and the blue beam is influenced by means of dynamic convergence. However, it will be evident that it is alternatively possible to cause overlapping of two other beams, dependent upon the positioning of the deflection coils. It is necessary only that the deflection in one of the two directions is parallel to the plane in which two of the three guns are located.

It will also be evident that, if the error in the x-direction cannot be reduced completely to zero, it is necessary to apply dynamic convergence also to the red and green beams. True, in this case, again three additional deflection coils are required, but the energy required for the convergency of the red and green beams may be considerably less than if use is made of deflection coils which are not built up in accordance with the invention.

What is claimed is:

1. A color cathode-ray tube comprising a substantially planar screen located substantially perpendicular to the axis ofand at one end of the tube, two electron guns arranged at the other end of the tube spaced from the tube axis and directed toward the screen and at which a straight line in a plane parallel to that of the screen joining the two guns is spaced from the axis, and a deflection coil system for deflecting the electron beams produced by the two guns in two orthogonal directions,

one of said orthogonal directions extending parallel to the said straight line joining two of the said guns, said coil system being operable to cause the beams from said two guns to form a linearly elongated cross-section whose longest dimension is perpendicular to said one deflection direction and which is located in a meridional image plane which substantially coincides with the plane of said screen for said one deflection direction, and to form a linearly elongated cross-section whose longest dimension is parallel to the other deflection direction and which is located in a sagittal image plane which substantially coincides with the plane of said screen for said other deflection direction.

2. A three-color cathode-ray tube comprising a substantially planar screen located substantially perpendicular to the axis of and at one end of the tube, three electron guns arranged at the other end of the tube and symmetrically around the tube axis and directed toward the screen and at which straight lines in a plane parallel to that of the screen joining the three guns form a triangle, a deflection coil system for deflecting the electron beams produced by the three guns in two orthogonal directions, one of said orthogonal directions extending parallel to the straight line joining two of the said guns in the parallel plane, said coil system being operable to cause the beams from said two guns to form a linearly elongated cross-section whose longest dimension is perpendicular to said one deflection direction and which is located in a meridional image plane which substantially coincides with the plane of said screen for said one deflection direction, and to form a linearly elongated cross-section whose longest dimension is parallel to the other deflection direction and which is located in a sagittal image plane which substantially coincides with the plane of said screen for said other deflection direction, and convergence means acting on the beam from the third gun in accordance with its deflection to cause it to coincide with the other two beams at the screen.

3. A three-color cathode-ray tube comprising a substantially planar screen located substantially perpendicular to the axis of and at one end of the tube, three electron guns arranged at the other end of the tube and symmetrically around the tube axis and directed toward the screen and at which straight lines in a plane parallel to that of the screen joining the three guns form an equilateral triangle,'and a deflection co-il system for producing a deflecting field within the tube for deflecting the electron beams produced by the three guns in two orthogonal directions, said tube axis corresponding to the z-axis of a rectangular system of coordinates, one of said orthogonal directions extending parallel to the x-axis and to the straight line joining two of the said guns in the parallel plane, the other orthogonal direction extending parallel to the y-axis, said deflection system producing within the tube a deflecting field having a y-component Hy in said one deflection direction, as measured in the plane x=0, determined by a power series of x of the form.

and an x-component H in the other deflection direction, as measured in the plane y=0, determined by a power series of y of the form and in which the integrals Zs Z, I H (zz.) dz andf H (z-z.) dz

are substantially zero for both field components, where the coeflicients H and H are positive and the coefficients H and H are functions of z, Z0 is the coordinate of the end of the deflection system remote from the screen and z, is the position of the screen in the coordinate system, the value of the terms H H =h for deflection in the y-direction at the end of said coil system adjacent said 11 screen being negative and at the beginning of said coil system being positive, the value of the term for deflection in the x-direction has the opposite polarities and h; having an absolute value that exceeds h wherein the coefficients H H H and H are determined by measuring the x-component H of the magnetic field in the plane x= at a particular y-point and substituting in the equation and by measuring the y-component H of the magnetic field in the plane y=0 at a particular x-point and substituting in the equation whereby the meridional image plane for said one direction of deflection produced by said deflecting field is substantially coincident with the surface of the screen, and the sagittal image plane for the other direction of deflection is also substantially coincident with the surface of the screen.

4. In combination; a cathode-ray tube having a substantially planar apertured screen extending substantially at right angles to the longitudinal axis of the tube and three electron guns arranged symmetrically around the tube axis for producing three electron beams directed toward the screen; and electromagnetic deflection means located between said electron guns and said screen for producing a deflecting field within the tube for deflecting said electron beams in two substantially perpendicular directions, whereby said beams may be caused jointly to scan said screen, one of said perpendicular directions extending parallel to a straight line in a plane parallel to the screen joining two of the guns, said deflection means including first and second ferromagnetic annular members surrounding the tube with the second member being closer to the screen than the first member, a first coil system for deflecting said beam in said one direction of deflection comprising a first set of four coils wound toroidally on the first annular member and a second set of four coils wound toroidally on the second annular member, a second coil system for deflecting said beam in the other direction comprising a third set of four coils Wound toroidally on the first annular member and a fourth set of four coils wound toroidally on the second annular member, two coils of each set of four coils being mounted substantially diametrically opposite one another and being connected so as to be traversed by deflection current in opposite senses, whereby said deflection means produce a deflection field in which the meridional image plane for said one direction is substantially coincident with the surface of said screen, and the sagittal image plane for the other direction of deflection is also substantially coincident with the surface of said screen.

5. The combination set forth in claim 4 wherein the first and second sets of coils produce a field having a magnitude equal to 3 +4 cos t/J which magnitude is predominately negative for the first set and predominately positive for the second set, and the third and fourth sets produce a field having a magnitude equal to COSZWI which latter magnitude is predominately positive for the third set and predominately negative for the fourth set wherein 0 is the angle between lines drawn from the first and second sets toward the tube axis and the said one deflection direction, 0 is the angle between lines drawn from the third and fourth sets toward the tube axis and the other deflection direction, and p and p; are the radii, respectively, of the first and second annular members.

6. The combination set forth in claim 5 wherein the coils of each set are connected in series each with a portion of an impedance and the two sets and their associated portion of the impedance are connected in parallel, said impedance being variable to control the relative magnitude of the current through the coil sets.

7. The combination set forth in claim 5 wherein the coils of both sets are connected in series, and all the coils are connected in parallel with a variable impedance, an intermediate portion of said impedance being connected to the junction of the two sets of coils.

8. The combination of claim 5 wherein means are provided for acting only on the beam from the third gun for converging it in accordance with its deflection to cause it to coincide at the screen with the other two beams.

9. In combination; a cathode-ray tube having a substantially planar apertured screen extending substantially at right angles to the longitudinal axis of the tube and three electron guns arranged symmetrically around the tube axis for producing three electron beams directed toward the screen; and electromagnetic deflection means located between said electron guns and said screen for producing a deflecting field within the tube for deflecting said electron beams in two substantially perpendicular directions, whereby said beams may be caused jointly to scan said screen, one of said perpendicular directions extending parallel to a straight line in a plane parallel to the screen joining two of the guns, said deflection means including a first pair of coil halves diametrically surrounding the tube for deflection in said one direction and a second pair of coil halves diametrically surrounding the tube between and symmetrically relative to the first pair for deflection in the other direction, the coils of said first pair having a shape producing a field at which the value measured at the coil end adjacent the screen is predominately positive but at the beginning of the coil is predominately negative, the coils of said second pair having a shape producing a field at which the value References Cited in the file of this patent UNITED STATES PATENTS 2,679,614 Friend May 25, 1954 2,719,249 Friend Sept. 27, 1955 2,747,134 Allwine May 22, 1956 2,751,519 Friend June 19, 1956 UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No, 2,945 157 July 12, 1960' Johan H aantjes et al0 It is hereby certified that erroi' appears in the above numbered patent requiring correctiohand that the said Letters Patent should read as coz rected below.

Column 3, line 19, before "point" insert from Signed and sealed this 4th day of July 1961.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissionel of Patents UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No. 2,945, 157 July 12, 1960 Johan Haantjes et a1 It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 19, before "point" insert from Signed and sealed this 4th day of July 1961.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No, 2,945, 157 July 12, 1960 Johan Haantjes et ale It is hereby certified that error appears in the above numbered patent requiring correctionand that the said Letters Patent should read as corrected below.

Column 3, line 19, before "point" insert from Signed and sealed this 4th day of July 1961.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents 

